While an optical recording medium such as a CD or DVD has widely been employed as a recording medium for recording digital data, recently a next generation of an optical recording medium having a larger capacity and a higher data transfer rate began to be developed extensively.
Such a next generation optical recording medium necessarily have an extremely reduced beam spot diameter of the laser beam employed for recording and reading the data for the purpose of increasing the recording capacity and achieving an extremely high data transfer rate.
In order to reduce the laser beam spot diameter, the numerical aperture (NA) of an object lens for converging the laser beam needs to be 0.7 or higher, for example, as high as about 0.85, while the wavelength (λ) of the laser beam needs to be 450 nm or less, for example as short as about 400 nm.
In addition, for the purpose of recording data on an optical recording medium with a reduced laser beam spot diameter while enabling a readout of the data from the optical recording medium, a coma aberration W represented by the following Equation (1) needs to be kept at a small value.
                    W        =                                            d              ·                              (                                                      n                    2                                    -                  1                                )                            ·                              n                2                            ·              sin                        ⁢                                                  ⁢                          θ              ·              cos                        ⁢                                                  ⁢                          θ              ·                                                (                  NA                  )                                3                                                          2            ⁢                                          λ                ⁡                                  (                                                            n                      2                                        -                                                                  sin                        2                                            ⁢                      θ                                                        )                                                            5                2                                                                        (        1        )            
In Equation (1), n is a refractive index of the light transmission layer, and θ is the deviation of the optical axis of the laser beam.
As evident from Equation (1), a reduction in the laser beam wavelength λ in combination with an increase in the object lens numerical aperture NA leads to a drastic increase in the coma aberration W and the coma aberration W is dependent on the optical axis deviation θ, and accordingly when using a laser beam of a short wavelength λ together with an object lens having a large numerical aperture NA a change in the optical axis θ leads to a substantial variation in the coma aberration W, resulting in a reduction in the tolerance toward the change in the optical axis θ, namely, the tilt margin.
Accordingly, for the purpose of reducing the laser beam wavelength λ and increasing the object lens numerical aperture NA while keeping the tilt margin at a level similar to that of a conventional optical recording medium such as a DVD, the thickness d of the light transmission needs to be reduced, and for example in the case employing a laser beam at a wavelength of about 400 nm and an object lens whose numerical aperture NA is about 0.85 the thickness d of the light transmission needs to be as small as 100 μm.
Therefore, unlike to a currently employed optical recording medium such as DVD, a next generation optical recording medium suffers from a difficulty in forming a layer such as a recording layer on a support substrate having a light transmissibility enabling an incidence of a laser beam, and eventually a method for forming as a light transmission layer having a light transmissibility enabling an incidence of a laser beam a resin layer as thin as about 100 μm on an information layer including a recording layer formed on the support substrate is proposed.
As a method for forming a light transmission layer whose thickness is about 100 μm, a method in which a resin film formed from a material having a light transmissibility is laminated on an information layer to form a light transmission layer or a method in which a resin composition solution is coated on an information layer by a spin coating method and an ultraviolet light or an electron beam is irradiated to effect a curing whereby forming a light transmission layer is proposed.
Also since a light transmission layer formed in a next generation optical recording medium is a layer enabling an incidence and a transmission of a laser beam as described above. Any scratch on its surface causes a diffused reflection of the laser beam which leads to a difficulty in effecting a recording of data on the information layer of an optical recording medium as desired and a readout from the information layer of the optical recording medium. Therefore, the light transmission layer needs to have a surface which is difficult to be scratched and is excellent in terms of the scratch resistance. At the same time, the light transmission layer needs to have a high light transmittance in the range of the wavelength of a laser beam employed for recording and reading the data, since the laser beam is focused on the information layer in the optical recording medium through the light transmission layer when the recording of data on the information layer of an optical recording medium and a readout from the information layer of the optical recording medium.
Nevertheless, an attempt to form a light transmission layer by laminating a resin film molded from a material having a light transmissibility on an information layer leads to an increased cost, since such a resin film having a light transmissibility and excellent optical characteristics is very expensive and such a formation of the light transmission layer by laminating the resin films suffers from a markedly reduced productivity.
On the other hand, when a light transmission layer is formed by applying a solution of a resin composition on an information layer and curing the resin composition by irradiating with an ultraviolet light or an electron beam, there has been a problem that the resin composition shrinks upon curing, resulting in a warp formed in an optical recording medium.
Especially, an attempt to form a light transmission layer using a resin composition having a high hardness in order to obtain a highly scratch resistant light transmission layer poses a need of using a resin composition having a high crosslinking density, which allows the resin composition to undergo a further increased shrinkage upon curing, resulting in a substantial warp formed in an optical recording medium.
Accordingly, in order to ensure the prevention of the warping of an optical record medium, it is required to form a light transmission layer using a resin composition having a reduced shrinkage upon curing.
Nevertheless, the formation of a light transmission layer using a resin composition undergoing a reduced shrinkage upon curing leads to a reduced hardness of the light transmission layer which allows the surface of the light transmission layer to be scratched readily, resulting in a problem of a reduced scratch resistance.
While such a problem can be solved by providing a protective layer such as a hard coat layer formed from a material having a high hardness on the surface of a light transmission layer, the formation of the protective layer such as a hard coat layer on the surface of the light transmission layer leads not only to an increased production cost of the optical recording medium but also a markedly poor productivity.
Accordingly, an object of the present invention is to provide a method for producing an optical recording medium by which a light transmission layer having excellent scratch resistance and light transmissibility can be formed by a spin coating method and by which an optical recording medium whose warping is suppressed effectively can be produced at a low cost and a high productivity. Another object of the present invention is to provide an optical recording medium having a light transmission layer having excellent scratch resistance and light transmissibility, wherein a warping of the optical recording medium is suppressed effectively.